The invention generally relates to a method and an apparatus for processing objects, in particular for rapidly processing dielectric substrates by use of laser beams.
Material processing using laser beams has increasingly gained in importance due to the rapid development of laser technology in recent years. In the field of electronic fabrication, in particular, due to the increasing miniaturization of the components, laser processing of printed circuit boards or substrates has become an indispensable tool in order to enable the microstructuring of components and/or substrates that is required on account of the miniaturization of the components. Thus, by way of example, it is possible to bore holes into substrates which have a diameter which is significantly smaller than the hole diameters of holes bored by conventional boring devices. Assuming that the laser power of the laser beam impinging on the substrate is known exactly, it is possible to bore not only through holes but also so-called blind holes. These are important particularly for multilayer printed circuit boards since, by uses of a subsequent metallization of a blind hole, different metallic layers of the multilayer printed circuit board can be electrically conductively connected to one another and the integration density on a substrate can thus be significantly increased.
U.S. Pat. No. 5,593,606 discloses a laser processing apparatus which can be used to bore holes having a diameter of between 50 and 200 xcexcm into multilayer substrates. The laser light source used is a continuously pumped, Q-switched Nd:YAG laser which, after frequency conversion, generates light pulses in the ultraviolet spectral region. The individual light pulses have a mean pulse power of approximately 250 mW and a pulse length of the order of magnitude of 100 ns. This results in a comparatively low energy of the individual light pulses of approximately 25 nJ, so that a multiplicity of laser pulses have to be used to bore a single hole. Moreover, since the pulse repetition frequency is limited to a few kHz, the throughput, i.e. the number of holes which can be bored per unit time, is correspondingly low, so that only a relatively small number of holes can be bored per unit time using this laser processing apparatus, depending on the material and the thickness of the layers that are to be bored through.
Furthermore, it is known that pulsed CO2 lasers having a wavelength of 9.2-10.6 xcexcm or pulsed solid-state lasers such as, for example, Nd:LYAG lasers or Nd:YVO4 lasers having a fundamental wavelength of 1064 nm can be used for material processing and, in particular, also for boring holes into substrates. The use of such conventional CO2 laser light sources emitting in the infrared spectral region has the disadvantage that the laser pulses generated are relatively long with a pulse length of the order of magnitude of xcexcs. As a result, the substrate to be processed is exposed to a high thermal loading, so that the geometry of the bored holes deviates considerably from the optimum (cylindrical or conical) form due to a boring burr or due to depositions at the edge of the hole and the quality of the bored holes is thus reduced. The depositions arise for example as a result of solidified vapor generated beforehand by sublimation from the substrate material on account of the heating by the laser beam. The deposition may, however, also comprise small grains of the solid substrate material which are centrifuged on account of a high degree of inhomogeneous heating of the substrate at the edge of the hole.
An embodiment of the invention is based on an object of providing a method and an apparatus for boring holes into dielectric substrates, in the case of which a multiplicity of high-quality holes can be bored within a short period of time.
An embodiment of the invention is based on the insight that given suitable parameters, i.e. the wavelength, the pulse lengths, the repetition rate and the pulse energy of the processed laser beam, it is possible to improve both the throughput, i.e. the number of holes bored per unit time, and the resulting hole quality. Medium infrared spectral region is understood hereinafter to mean a wavelength of 1.2 xcexcm to 7 xcexcm and far infrared spectral region is understood to mean a wavelength of 7 xcexcm to 1000 xcexcm.
In accordance with one embodiment, a Q-switched CO2 laser which emits light with a wavelength of approximately 9.2 xcexcm is suitable for generating the pulsed laser beam. The Q-switching of the CO2 laser can be realized by use of a so-called acousto-optical switch. By way of example, a CdTe crystal excited to effect mechanical oscillations with a frequency in the MHz range is suitable for this purpose.
Focusing of the processing laser beam onto a diameter of approximately 50 to 200 xcexcm can be realized in particular when the laser beam emitted by the laser light source is widened by use of a beam expansion upstream of the actual focusing optical arrangement. It is pointed out that the use of a beam expansion results in a lower depth of field of the laser beam to be processed, so that the distance between the focusing optical arrangement and the object surface to be processed must be complied with, with the highest possible accuracy. Undesirable widening or conical geometries of the bored holes can be avoided in this way.
The boring of so-called blind holes is used in particular when processing multilayer substrates.
Depending on the substrate material and the thickness or the depth of the hole to be bored, the hole can be bored either by use of a single laser pulse or by use of a sequence of laser pulses that are directed successively onto the object to be processed. When using a sequence of a plurality of laser pulses, it must be taken into account that, in order to avoid a poor hole quality, the individual laser pulses impinge as far as possible at the same location on the object. Experiments that have been carried out have revealed that a spatial overlap of the resulting focus areas of at least 66% should be complied with for this purpose.
In accordance with one embodiment, pulses having a length of at most 150 ns are preferably used for the processing of substrates made of the substrate material LCP (Liquid Cristalline Polymer), which has outstanding electrical properties right up to frequencies of 40 GHz and which is virtually impermeable both to liquid, oxygen and to other gases and liquids.
In accordance with one embodiment, a pulse repetition frequency of at least 50 KHz, preferably a pulse repetition frequency of between 60 KHz and 100 KHz, is suitable for processing the dielectric substrate FR4 (Flame Retard 4) that is mechanically reinforced with a glass fiber material, such as the material C-1080 from the company ISOLA, by way of example.
In accordance with one embodiment, the processing of an epoxy material, such as the material RCC (Resin Coated Copper) that is often used as a standard substrate in electronic fabrication, by way of example, requires a pulse repetition frequency of at least 80 KHz and preferably a pulse repetition frequency of approximately 100 KHz.